1. Field of the Invention
The present invention relates to an image forming apparatus with the so-called cleanerless configuration in which post-transfer residual toner that remains adhering to the surface of the latent image carrier after transfer of the toner image on the latent image carrier to the transfer member is recovered within the developing device.
2. Description of the Related Art
Normally, in an electrophotographic image forming apparatus, images are formed by the following process. First, an electrostatic latent image is formed on a latent image carrier, such as a photosensitive member or the like, that has been uniformly charged, by scanning the latent image carrier with light. Next, the toner image obtained by developing is either directly transferred from the latent image carrier to the recording medium, such as transfer paper or the like, or transferred from the latent image carrier to the recording medium, such as transfer paper or the like, via an intermediate transfer member.
The apparatus disclosed in for example Japanese Patent Application Laid-open No. 2003-316202 is known as an image forming apparatus of this configuration. In this image forming apparatus a toner image formed on the surface of a rotating drum shaped photosensitive member, which is the latent image carrier, is transferred in the primary transfer operation to an intermediate transfer belt at the primary transfer nip where the photosensitive member contacts the intermediate transfer belt. Then, after passing the primary transfer nip, the surface of the photosensitive member is uniformly charged by a charging device. Post-transfer residual toner adheres to the surface of the photosensitive member after passing through the primary transfer nip, but after uniformly charging the surface of the photosensitive member without removing the residual toner, the residual toner is recovered within the developing apparatus, which is the so-called cleanerless configuration.
When the transfer of the toner image from the photosensitive member to the transfer member is properly carried out, virtually all the post-transfer residual toner adhering to the surface of the photosensitive member after passing through the primary nip is oppositely charged toner, which is charged with the opposite polarity to the regular polarity. Therefore the inventors of the present invention made a prototype image forming apparatus using the cleanerless configuration as follows. A bias having the same polarity as the regular polarity of the toner is applied to an electrically conducting brush that contacts the photosensitive member after passing the primary transfer nip. This produces an image forming apparatus in which the oppositely charged toner, which is the majority of the post-transfer residual toner, is trapped within the brush. The post-transfer residual toner trapped within the electrically conducting brush is again transferred from the brush to the photosensitive member by changing the bias conditions immediately after completing the print job, or during the timing between feeding paper in continuous printing, or the like, to achieve recovery within the developing device.
However, it is known that in this configuration, unevenness of the density of halftone images occurs due to the transfer efficiency of the toner image from the photosensitive member to the transfer member. Also, it is known that this unevenness of the density is caused as follows. When the transfer of the toner image from the photosensitive member to the transfer member is properly carried out, the amount of residual toner is comparatively small, virtually all of which is oppositely charged toner. On the other hand when the transfer efficiency is comparatively poor, the amount of post-transfer residual toner is comparatively large, and includes regular polarity toner which is regularly charged in addition to the oppositely charged toner. The regular polarity toner is not trapped by the electrically conducting brush, but remains on the developing area. If the place on the photosensitive member to which the regular polarity toner is adhering when it is returned to the developing area is a comparatively large background area, then the toner is transferred to the developing roller or the like at the developing area and recovered in the developing device, and therefore this is not a big problem. However, if the place on the photosensitive member that is returned to the developing area with regular polarity toner adhering is a small area background portion formed between dots of a halftone image, the toner is affected by the electric field of the surrounding latent image that is in dot form, and remains as it is on the surface of the photosensitive member. In this way, the problem arises that the halftone image is denser in parts.